CS490ns Firewalls Sp11 Bw

8/10/2019 CS490ns Firewalls Sp11 Bw

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FirewallsWhat they do.

How they work.


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Outline

What is a firewall? Architectures

Stand Alone / application / proxy Personal / host based Gateway / packet filters Enterprise / hardware

Roles Bastion DMZ

Packet Filtering concepts IPTables Stateful filtering

Packet Forwarding Ethernet bridge


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What is a Firewall?

A hardware or software device that monitors (andcontrols ?) the transmission of packets that attempt topass through the perimeter of a network (or host).

Provide 2 basic security functions Packet Filtering

Application Proxy gateways

Additional security features Log unauthorized (and authorized ?) access attempts

Provide VPN Connections Support user authentication

Shield internal machines from outside view


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What should a firewall do?

Control the flow of packets to/from Internet

Block external login as root (?)

Must distinguish between local andInternet packets (even spoofed addresses)

Support limited user accounts

Log all system activities


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Types of Firewalls

Stand Alone / application / proxy

Enterprise / Local

Hardware / Software Gateway / router / packet filter

Personal / host based

Windows firewallincoming protection ZoneAlarm, Linux, etc.incoming / outgoing

filter


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Types of Firewalls

Internet

Corporate

Network

Router /packet filter

StatefulFirewall

ApplicationProxy

Host-basedFirewall


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Standalone Proxy Firewalls

Application Gateways Intended to buffer the interface betweenan internal application and the Internet Web Servers

Mail Servers File Transfer

Controls flow of packets into and out of

local network Limit access to specific web sites Cache results for use by other internal hosts

Hide internal IP addresses from network view


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Enterprise Firewalls

Intended to support larger traffic volumes

Provides more sophisticated support

Stateful filtering, etc. Software

Checkpoint Firewall 1, Microsoft ISA, SemanticEnterprise, etc.

Hardware Cisco PIX, SonicWall, Watchguard, etc.

Expensive!


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Gateway / Packet Filter

May be embedded in sophisticated routers

May be used for SOHO networks

May be incorporated into small SOHO routers May be incorporated into a gateway host(Linux ?)

Provides the ability to monitor and controlpackets through the gateway / router.

Generally support in / out / through filtering

May not include stateful filtering capabilities


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Host-based Firewalls

Intended as a last line of defense for thehost computer

Runs as a background process on host

Limited bandwidth available

Generally supports incoming port filtering

Can specify which ports (if any) can support

incoming connection requests.

Occasionally supports outgoing filtering(looking for worms, trojans, etc.)


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Firewall Roles

Bastion Hosts

Hardened systems that typically run a firewalland perhaps an application as well

DMZdemilitarized zone

An isolated subnetwork that includes allservices that are offered over the internet

(and perhaps to the internal network as well).


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Bastion Firewall and Host

LAN

Internet

Firewall

WebServer


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DMZ

LAN

Internet

Web E-mail

DMZ


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What is Packet Filtering?

The process of deciding which packets to allowthrough the filter, based on attributes of thepacket Source / Destination Port

Source / Destination IP Address

Status flags in the packet (syn)

Originating protocol (icmp, tcp, etc.)

Connection state (tcp)

Linux (2.4+) supports Netfilter (based oniptables)


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How does Packet FilteringWork?

Define rules to allow or block specific types ofpackets

Firewall screens all packet headers to look for

matches against the rules Apply rules in the order in which they are stored

Allow or block packets based on rule matches.

If a packet matches no rules, apply default

behavior to the packet (usually deny).


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Packet Filtering Issues

Rules are complex. Easy to introduce errors

Filters based on IP addresses. If authorizedsites are hacked, your site is compromised

IP Spoofing can fake authorized (internal?) sites. Routers can be hacked to reroute internal

packets

Activities need to be logged

Internal host adresses should be hidden


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Iptables

Administration tool for IPv4 packet filteringand NAT

Used to set up, maintain, and inspect thetables of IP packet filtering rules used bythe kernel to manage packet flow throughthe firewall.

Based on tables that specify the overalltask and chains that identify the position ofthe packet in the packet flow.


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IPTables tables

Filter table Used to control the flow of packets based on packet attributes

Only filter packets, dont modify packets here.

Network Address Translation (NAT) table Used to change the source / destination IP address and / or port

of selected incoming / outgoing packets

Mangle table Supports specialized packet handling / routing

Change contents of packet

Experimental and developing tables


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Basic Packet Filtering

Internet

LAN

Input

Output

Forwardfiltertable

RH-Firewall-1-INPUT


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Incoming Packets to Filter

Illegal Incoming Source IP Addresses Your IP Address Your LAN Address Private Network Addresses Multicast IP Addresses

Loopback Interface Addresses Nuisance sites / networks Remote Source Port Filtering Local Destination Port Filtering

Incoming TCP connection-state filtering Probes and Scans DoS Attacks Etc.


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Packet Filtering alert list

CERT www.cert.orgCarnegie-Mellon Software Engrg Inst.

www.us-cert.gov

Port Filter List (3/08) DNS zone transfers 53 tftpd 69

link 87

RPC / NFS 111 / 2049

BSD r commands 512, 513, 514 lpd 515

uucpd 540

openwindows 2000

X windows 6000 +
http://www.cert.org/http://www.us-cert.gov/http://www.us-cert.gov/http://www.us-cert.gov/http://www.us-cert.gov/http://www.cert.org/


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Outgoing Packets to Filter

Why? Consideration for fair use in Internet

Distribution of private information

Detection of unwanted client programs (Trojans, etc.) See http://www.us-cert.gov/cas/tips/ST06-001.html

What Legitimate, routable addresses only

Destination IP Addresses

Destination ports

Source Ports


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Filter Table Chains

May be associated with any interface (eth0, etc.)

INPUT

Used to test packets that come into the firewall

OUTPUT Used to test packets that are leaving the firewall

FORWARD

Used to test packets that are passing through thefirewall

Packets should pass through only 1 chain


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Filter table packet flow

Routing

InputChain

ForwardChain

OutputChain

Drop

Drop Drop

LocalProcessesInputChain

Drop


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Iptables rule structure

Iptablest table action chain rule target

Which table are we working with (filter is default)

What action do we want to do to that table (insert,

delete, etc.) Which chain in that table are we working with

What do we want to do?

Where do we go if we match the rule?


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IPTables Actions

Create a new chain (-N).

Delete an empty chain (-X).

Change the default policy for a chain. (-P). List the rules in a chain (-L).

Flush the rules out of a chain (-F).

Zero the packet and byte counters on allrules in a chain (-Z).


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IPTables Actions

Append a new rule to the end of a chain (-A).

Insert a new rule at some position in a chain (-I).

Replace a rule at some position in a chain (-R). Delete a rule at some position in a chain, or the

first that matches (-D).


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IPTables targets

ACCEPT Stop processing and pass to application / OS

DROP Stop processing and block packet

LOG

Packet info sent to syslog. Continue processing REJECT

Stop processing and send reject message to source

DNAT Change destination network address

SNAT Change source network address

MASQUERADE Do source network address translation (PAT)


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Example Filter Rules

#Allow traffic on the loopback interface

iptablesA INPUTi loj ACCEPT

iptablesA OUTPUTi loj ACCEPT

# Set Default policy for chain Iptables --policy INPUT DROP

#Allow all outgoing connections

iptables -A block -m state --state NEW -i ! ppp0 \

-j ACCEPT #Block incoming attempts to Xwindows

iptablesA INPUTi eth1 -p tcp --syn \--destination-port 6000-6003 -j REJECT


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Example Filter Rules

#Allow incoming connections to local web serverIptablest filter -A blockp tcp --dport 80 --i eth1 \

-j ACCEPT

#Insert a rule that allows incoming udp packets to port 12345

iptablesI block 7p udpdport 12345j ACCEPT#Allow DNS requests NOT from outside

iptables -A blockp tcp --dport 53 -m state --state NEW \-i ! eth1 -j ACCEPT

#Allow (and redirect) incoming web connections to 192.168.5.6

iptablest natA PREROUTINGd eth1 -p tcp \--dport 80 -j DNAT --to-destination 192.168.5.6


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Simple Firewall table

## Insert connection-tracking modules (not needed if built into kernel).

insmod ip_conntrack

insmod ip_conntrack_ftp

## Make chain that blocks new connections, except if coming from LAN.iptables -N block

iptables -A block -m state --state ESTABLISHED,RELATED -j ACCEPT

iptables -A block -m state --state NEW -i ! ppp0 -j ACCEPT

iptables -A block -j DROP

## Jump to that chain from INPUT and FORWARD chains.

iptables -A INPUT -j block

iptables -A FORWARD -j block


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Iptables default config file/etc/sysconfig/iptables

# Firewall configuration written by system-config-securitylevel# Manual customization of this file is not recommended.*filter:INPUT ACCEPT [0:0]:FORWARD ACCEPT [0:0]:OUTPUT ACCEPT [0:0]:RH-Firewall-1-INPUT - [0:0]

-A INPUT -j RH-Firewall-1-INPUT-A FORWARD -j RH-Firewall-1-INPUT-A RH-Firewall-1-INPUT -i lo-j ACCEPT-A RH-Firewall-1-INPUT -p icmp --icmp-type any-j ACCEPT-A RH-Firewall-1-INPUT -p 50-j ACCEPT-A RH-Firewall-1-INPUT -p 51-j ACCEPT-A RH-Firewall-1-INPUT -p udp --dport 5353 -d 224.0.0.251-j ACCEPT

-A RH-Firewall-1-INPUT -p udp -m udp --dport 631-j ACCEPT-A RH-Firewall-1-INPUT -p tcp -m tcp --dport 631-j ACCEPT-A RH-Firewall-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 22-j ACCEPT-A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibitedCOMMIT


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CentOS 5.5 Firewallpart 1

[rcotter@lserver3 ~]$ sudo iptables -LChain INPUT(policy ACCEPT)target prot opt source destination

RH-Firewall-1-INPUT all -- anywhere anywhere

Chain FORWARD(policy ACCEPT)target prot opt source destinationRH-Firewall-1-INPUT all -- anywhere anywhere

Chain OUTPUT(policy ACCEPT)target prot opt source destination


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CentOS 5.5 Firewallpart 2Chain RH-Firewall-1-INPUT (2 references)target prot opt source destination

ACCEPT all -- 0.0.0.0/0 0.0.0.0/0ACCEPT icmp -- 0.0.0.0/0 0.0.0.0/0 icmp type 255ACCEPT esp -- 0.0.0.0/0 0.0.0.0/0ACCEPT ah -- 0.0.0.0/0 0.0.0.0/0ACCEPT udp -- 0.0.0.0/0 224.0.0.251 udp dpt:5353ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 udp dpt:631

ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:631ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHEDACCEPT tcp -- 192.168.1.0/24 0.0.0.0/0 state NEW tcp dpt:22ACCEPT tcp -- 134.193.12.34 0.0.0.0/0 state NEW tcp dpt:22ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 state NEW udp dpt:137ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 state NEW udp dpt:138

ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:139ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:445ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 state NEW udp dpt:2069ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:3128ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:3306REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited


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Filter table

Input, forward chains point to custom chain RH-Firewall-1-INPUT

Output chain set to accept all (allow any outgoing traffic)

RH-Firewall-1-INPUT chain Initial 4 rules allow broad classes of packets

Allow multicast DNS

Allow ipp (Internet Printing protocol)

Allow incoming UDP packets to port 12345 Special server set up for cs423 class

Allow incoming SSH connections

Reject everything else!


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Network Address Translation

What? Translates IP addresses and / or ports as

packet passes through firewall

Only first packet of a connection will traversethe table. All remaining packets are modifiedthe same as the first packet.

Why?

Private local IP Addresses Multiple Servers (load sharing)

Transparent Proxying


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NAT table

Used to map local IP addresses to a set ofroutable addresses (NAT)

Used to map local IP addresses to a set of portsassociated with a single routable address

(NAPT) Used to map local IP addresses to a set of ports

associated with a variable routable address(masquerade)

Dial-up connection Dynamically assigned IP address

Other


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NAT

Two types of NAT

Source NAT (snat) used to translate thesource IP address of a packet (typically

outgoing) Destination NAT (dnat) used to translate the

destination IP address of a packet (typicallyincoming).


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NAT table chains

Pre-routing

Used to test / modify the destinationaddresses of incoming packets

Output Used to change the source (or destination)

address of locally generated packets

Post-routing Used to change the source address of

outgoing packets.


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Simple NAT table rules

# Masquerade out ppp0

iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

# Disallow NEW & INVALID incoming or forwarded packets from ppp0.

iptables -A INPUT -i ppp0 -m state --state NEW,INVALID -j DROP

iptables -A FORWARD -i ppp0 -m state --state NEW,INVALID -j DROP

# Turn on IP forwarding (in RAM)

echo 1 > /proc/sys/net/ipv4/ip_forward# Turn on IP forwarding (in file /etc/sysctl.conf)

net.ipv4.ip_forward = 1


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Mangle table

Used for special routing and packetmodification.

Use TOS (type of service) field in IP header.

TTL

Can be used to set and test markers placed


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Mangle Table Routing

AS

Internet


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Linux Firewall Mgmt

iptables Make changes to memory image of firewall rules

iptables-save

Display a copy of the memory image Can redirect the copy to a file using output redirection

Iptables-save > /etc/sysconfig/iptables

iptables-restore

Rebuild memory image from keyboard or file (using redirection) Security Level and Firewall Applet (Fedora)

Creates an automatic backup file: /etc/sysconfig/iptables


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IPTables Constraints

Based on IPonly

Dont run IPX, appletalk, etc. as these protocols are

not filtered

Packets traversing the filter table will passthrough only 1 chain


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Port Forwarding

Internet

LAN

123.234.56.78:80192.168.3.6:80

HTTPD

SOHO R t P t R


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SOHO Router Port RangeForwarding


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IPTables Port Forwarding

For incoming packets iptables -t nat -A PREROUTING -p tcp -d --dport -j DNAT --to-destination:

For returned packets iptables -m conntrack --ctstate DNAT -t nat -A

POSTROUTING -p tcp -d --dport -j SNAT --to-source

For packets originating on firewall iptables -t nat -A OUTPUT -p tcp -d --

dport -j DNAT --to-destination:


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IPtables rerouting Issues

Often, when we re-route packets, we onlyneed to change destination (or source) IPaddress.

Sometimes (if we are rerouting to a locallyconnected destination) we need to changeboth IP address and MAC address.

IPtables only filters IP traffic. It cannotchange IPX, netBEUI, Appletalk, etc.


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EBtables

Ethernet Bridge tables Intended to support filtering of packets that

IPtables cannot filterEthernet protocol,

MAC address, ARP, netBEUI, IPX, etc. Basically adds nonIP filtering.

802.1Q VLAN filtering

MAC address NAT

Frame counters

Linux bridge-nf code Passes bridged traffic to IPtables


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EBtables Structure

broutetable BROUTINGchain

Choose whether to process packet at layer 2(bridge) or at layer 3 (route)

e.g. route normal IP traffic and bridge IPX traffic

filtertable FORWARD, INPUT, OUTPUTchains

Route packets based on MAC addresses

nattable PREROUTING, OUTPUT, POSTROUTINGchains

Change MAC addresses (redirect based on MAC)


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Ethernet Bridge Firewall

Internet

LAN

Linux boxconfigured as a

bridge, with firewallinstalled


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Use bridging firewall (ebtables) to set uprules to pass packets through host.

Since processing happens at Data Link Layer,

there is no need to assign an IP address tohost interfaces, so machine is invisible tonetwork scanning.

Offers better protection, and lessconfiguration of the remaining network.

Can also be configured with an IDS.


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Create a virtual Ethernet bridge interface brctl addbr br0

Add our interfaces to the bridge brctl addif br0 eth0

brctl addif br0 eth1 Remove the IP configuration from interfaces

Ifconfig eth0 down

Ifconfig eth1 down

Ifconfig eth0 0.0.0.0 up Ifconfig eth1 0.0.0.0 up

Configure access for the bridge Local console, OOB network, configure 1 IP


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Ethernet Bridge Firewall (2)

Internet

LAN


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Example Firewall Application

Monitor all outgoing Traffic Most firewalls only monitor incoming traffic by

default

Identify what traffic is desired and blockthe rest. Many applications generate queries to their

servers

Spyware Hacks


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App development process

Capture all outgoing traffic Monitor traffic as it enters or leaves the

network(Ethernet Bridge)

Use iptables to log traffic. -A firewall-win1j LOGlog-level 4log-prefixWin1 log-tcp-optionslog-ip-options

Set up syslog to divert level 4 messages to a

separate file (see syslog.conf) kern.warning /var/log/iptables.log

Save data daily to separate file iptables_log_022011

# Generated Manually 8/19/10*filter


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Primary

FirewallFilterTable

:INPUT ACCEPT [0:0]

:FORWARD ACCEPT [8183:1429550]

:OUTPUT ACCEPT [14722:762210]

-N RH-Firewall-1-INPUT

# Create separate chains for each host - 8/19/10

-N Firewall-Win2

-N Firewall-Win1

-N Firewall-lserver3

#new line 8/26/10 - start monitoring this machine

-N firewall-bridge

-A OUTPUT -j firewall-bridge

-A INPUT -j RH-Firewall-1-INPUT-A FORWARD --src 192.168.1.25 -j Firewall-lserver3

-A FORWARD --src 192.168.1.35 -j Firewall-Win2

-A FORWARD --src 192.168.1.30 -j Firewall-Win1

-A RH-Firewall-1-INPUT -i lo -j ACCEPT

-A RH-Firewall-1-INPUT -p icmp -m icmp --icmp-type any -j ACCEPT

-A RH-Firewall-1-INPUT -p esp -j ACCEPT

-A RH-Firewall-1-INPUT -p ah -j ACCEPT

-A RH-Firewall-1-INPUT -p udp -m udp --dport 631 -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp --dport 631 -j ACCEPT

-A RH-Firewall-1-INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

-A RH-Firewall-1-INPUT -j DROP

-A Firewall-Win1 --dst 192.168.1.0/24 -j ACCEPT

A Firewall Win1 p icmp m icmp icmp type any j ACCEPT


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Win1OutgoingFirewall

Chain

-A Firewall-Win1 -p icmp -m icmp --icmp-type any -j ACCEPT

-A Firewall-Win1 --dst 134.193.123.45 -j ACCEPT

-A Firewall-Win1 --dst 208.67.222.222 -j ACCEPT

# Allow queries to Dropbox


# Allow queries to Kapersky-A Firewall-Win1 --dst 38.117.98.0/24 -j ACCEPT




# Allow queries to Microsoft (update)





# Allow queries to dyndns.org



-A Firewall-Win1 --dst 208.78.69.0/24 -j ACCEPT# Lots of multicast traffic. Drop it.

-A Firewall-Win1 --dst 224.0.0.0/8 -j DROP

# Now, log everything else before dropping it

-A Firewall-Win1 -m physdev --physdev-in eth1 -j LOG --log-level 4 --log-prefix "Win1 " --log-tcp-options --log-ip-options

#For everything else, reject the traffic.

-A Firewall-Win1 -j DROP


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Capture Outgoing Traffic

Data Record1 per packet

Feb 19 00:01:03 bridge kernel: Win1 IN=br0OUT=br0 PHYSIN=eth1 PHYSOUT=eth0

SRC=192.168.1.35 DST=66.94.233.186LEN=40 TOS=0x00 PREC=0x00 TTL=128ID=10570 DF PROTO=TCP SPT=2323DPT=80 WINDOW=65185 RES=0x00 ACK

FIN URGP=0 Records per day ~ 40k to 80k+

P t S Att k D t t


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Port Scan Attack DetectorPSAD

Can be configured to detect various networkscans, invalid traffic, attacks, etc.

Can be used to fingerprint source machines Can be configured to provide active response

based on type of input, numbers of input packetsfor a predetermined period.

Can be used to sort and organize logged data.


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Summarize traffic

psad -m /var/log/iptables/iptables_log_022011 --gnuplot --CSV-fields dst src dp:count --gnuplot-graph points --gnuplot-xrange 0:100 --gnuplot-file-

prefix test_022011 test_022011.dat 1, 172, 2 ### 1=12.29.100.148 172=192.168.1.35

:

39, 172, 96 ### 39=66.94.233.186 172=192.168.1.35 :

246, 171, 1 ### 246=216.191.247.139 171=192.168.1.30


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Sort Traffic by Source

Use script (bash / awk / py / ?) to sort traffic intoseparate files by source

Use DNS to get domain name for sites

Win1_022011.lst 12.29.100.148: Output was 0 :

66.94.233.186: r3.ycpi.vip.mud.yahoo.net.

: 216.137.43.236: server-216-137-43-236.dfw3.cloudfront.net.


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Analyze traffic

Are addresses identifyable?

Is the traffic known / expected?

Why is traffic there?


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References

Firewalls and VPNsPrinciples and Practices Richard Tibbs / Edward OakesPrentice Hall2005

Linux Firewalls2nded.

Robert SieglerNew Riders Publishing2002 Guide to Firewalls and Network Security

Greg HoldenThomson/Course Technology2004

EBtables/IPtables Interaction on a bridge - 2003

ebtables.sourceforge.net/br_fw_ia/br_fw_ia.html

Red Hat Fedora Linux Secrets

Naba BarkakatiWiley - 2005


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Summary

What is a firewall? Architectures

Stand Alone / application / proxy Personal / host based

Gateway / packet filters Enterprise / hardware

Packet Filtering concepts Packet Forwarding

Roles Bastion DMZ

EBtables

CS490ns Firewalls Sp11 Bw

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